It is known for hydraulically operated camshaft phasers to be fitted with a locking system to control the position of the phaser when there is insufficient oil supply pressure to do so. An example of such a system is disclosed in GB 0428063.2. Conventionally, the locking system holds the phaser at one extreme of its operating range such that it will be returned to the locking position either by the camshaft drive torque, or by a simple return spring arrangement.
It has also been proposed in the prior art (see for example GB 2372797) to lock the phaser in an intermediate position, as this allows better optimisation of the engine start-up position for the phaser. However, the prior art only discloses the use of a spring to bias the phaser towards the intermediate position in which it can be locked by the locking system.